Indium tin oxide film (ITO film), as an important kind of transparent conductive film due to its excellent conductivity and visible-light transmittance, is widely used in photoelectric devices. At present, the ITO film is fabricated generally by using magnetron sputtering technique. In the magnetron sputtering technique, the plasma discharging is implemented under vacuum conditions, the movements of charged particles are controlled through electromagnetic field so that a target is bombarded by the moving particles, and atoms of the target are knocked off and attached to the substrate, thereby a desired thin film is formed on the substrate.
Nowadays, it is necessary for a Twist Nematic (TN) type liquid crystal display panel to have an ITO film formed on the color film substrate as a common electrode. However, the ITO film formed by using the magnetron sputtering technique has defects on film surface, for example, pin holes are likely to appear on the film surface. The cause and process of producing the pin holes are illustrated in FIGS. 1 and 2. During film coating, if there is a foreign matter 21 (e.g. dust, foreign particle and the like) on the color film substrate 1, a bump will be formed on the ITO film surface after film coating. In subsequent cleaning process, the bump is removed by washing, and a pin hole 22 is produced passing through the ITO film 2. The pin hole 22 will be displayed as an abnormal spot when the cell-forming process of the display panel is finished, which will negatively impact on the qualified rate of the liquid crystal display panel. Obviously, it is necessary to avoid forming the pin hole 22. An existing method for reducing the occurrence rate of the pin holes 22 is that a pre-cleaning process is performed before the film coating process, and the devices are all sealed and cleaned periodically before the film coating process. However, it is obvious that such method can only reduce the foreign matters on the substrate before the film coating process, and can hardly eliminate the foreign matters completely. Furthermore, in the liquid crystal display panels, a voltage should be applied to the common electrode so as to control the switch between dark and light for the pixel units. Thus, in order to ensure that the common electrode can withstand a desired voltage, the ITO film acting as the common electrode should have a certain thickness. Presently, the thickness of the ITO film on the color substrate in the TN type liquid crystal display panel is generally 150 nm. When fabricating the ITO film 2 with a thickness of 150 nm, the power of the film-coating equipment is 14.6 kW, which corresponds to a production time of 35 second per product. However, in actual production, when the film-coating power is higher than 8 to 9 kW, the probability that knobbles (protuberances) are generated on the surface of the target and on the ITO film 2 on the color substrate increases dramatically. The knobbles on the ITO film 2 are apt to form pin holes. Furthermore, the knobbles on the surface of the target are apt to cause point discharge so that the surface of the ITO film 2 formed by film coating protrudes to form a protruding part which cannot be removed by washing and should be removed by polishing during repairing process, thus deteriorating the quality of film coating, and resulting in undesirable values of film-surface resistance and transmittance. Further, due to these effects, the production line has to be suspended so as to polish and wash the surface of the target to remove the knobbles periodically. Consequently, the maintenance time for the equipments increases greatly, and the utilization, which means the percentage of time spent on creating value by the equipment in a planned time, is reduced.
When the film layer is formed by using other processes (e.g., the evaporating process, the sputtering process, the chemical vapor deposition process, the spraying process, and so on), the phenomenon of defective film surface caused by the foreign matters also occurs, and thereby the performance of subsequent products is affected.